Comparison of the mechanism of isoproterenol-stimulated glycogenolysis in skeletal muscle of normal and phosphorylase kinase-deficient mice (I strain).

Diaphragm extracts from mice with the phosphorylase kinase deficiency mutation (I strain) have only 3.7% of the phosphorylase kinase activity of muscle extracts of the control strain (C57BL). Nevertheless, previous studies have shown that isoproterenol-stimulated glycogenolysis in I strain diaphragm muscle at a rate 57% (average relative response at 10 isoproterenol concentrations) of that of C57BL (GROSS, S.R., MAYER, S.E. and LONGSHORE, M.A.: J. Pharmacol. Exp. Ther. 198: 523-538, 1976). The present studies were initiated to compare the mechanism of isoproterenol-stimulated glycogenolysis in I and C57BL diaphragms. Isoproterenol was found to stimulate phosphorylase b to a conversion with an EC50 of 8 nM in muscles from mice of either strain, and the maximum increase in phosphorylase alpha activity in I diaphragms was 23% of that in C57BL diaphragms. Moreover, the initial rate of increase in phosphorylase alpha activity in I diaphragms incubated with 40 nM isoproterenol was 24% of that in C57BL muscles. The isoproterenol-stimulated increases in cyclic AMP content in diaphragms of the two strains were the same. Incubation of I diaphragms with isoproterenol did not significantly increase the concentrations of AMP, IMP or inorganic phosphate, activators of phosphorylase beta activity, nor was there a decrease in ATP and glucose 6-phosphate content, allosteric inhibitors of phosphorylase beta activity. Thus, phosphorylase alpha formation is the principal, if not only, catalyst of isoproterenol-stimulated glycogenolysis in skeletal muscle of phosphorylase kinase-deficient mice, and no evidence was obtained indicating that allosteric regulation of phosphorylase beta activity is part of the mechanism.